Superconducting structure

ABSTRACT

A superconductive structure including a dielectric oxide substrate, a thin buffer layer of a superconducting material thereon; and, a layer of a rare earth-barium-copper oxide superconducting film thereon the thin layer of yttrium-barium-copper oxide, the rare earth selected from the group consisting of samarium, gadolinium, ytterbium, erbium, neodymium, dysprosium, holmium, lutetium, a combination of more than one element from the rare earth group and a combination of one or more elements from the rare earth group with yttrium, the buffer layer of superconducting material characterized as having chemical and structural compatibility with the dielectric oxide substrate and the rare earth-barium-copper oxide superconducting film is provided.

[0001] This invention was made with government support under ContractNo. W-7405-ENG-36 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

[0002] The present invention relates to superconducting structures andto a method of improving superconducting properties of selected(RE)Ba₂Cu₃O₇ films, where RE is a selected rare earth element.

BACKGROUND OF THE INVENTION

[0003] Since the discovery of high-temperature superconducting (HTS)materials (superconducting above the liquid nitrogen temperature of 77K) there have been efforts to research and develop various technologyand engineering applications using such HTS materials. In thin filmsuperconductor devices, the most progress has been made with fabricationof Josephson junctions and microwave devices utilizing an oxidesuperconductor including yttrium, barium, copper and oxide in thewell-known basic composition of YBa₂Cu₃O_(7-x) (hereinafter referred toas Y123). At liquid nitrogen temperatures and in high magnetic fields,the J_(c) of Y123 is superior to those of the bismuth, thallium andmercury based HTS materials. Thus, Y123 has generally been the preferredmaterial for thin film and bulk applications.

[0004] Even though Y123 is the material of choice for HTS applications,it still has a few problems. One problem is that Y123 has one of thelowest T_(c)'s among (RE)Ba₂Cu₃O_(7-x), materials (hereinafter referredto as RE123) which can limit J_(c) at the liquid nitrogen temperature(since J_(c) depends on T_(c); J_(c)≈(1−T/T_(c))^(3/2)). Another problemis that compared with SmBa₂Cu₃O₇ (Sm123) and NdBa₂Cu₃O₇ (Nd123) films,Y123 films have a much rougher surface morphology which is detrimentalfor any device application and imposes a materials challenge. Stillanother problem is that it has been known that Nd123 has a larger J_(c)in high magnetic fields than Y123. Hence, it has been important tocontinue development of RE123 films for various HTS applications.Previously, there have been some efforts to fabricate high quality RE123films. Sm123 and Nd123 films with good superconducting properties(T_(c)>90 K and J_(c)>10⁶ Amperes per square centimeter (A/cm²)) andsmooth surface morphology have been fabricated using Pulsed LaserDeposition (PLD), Molecular Beam Epitaxy (MBE), and Coevaporationtechniques. However, the optimization of film properties has only beenachieved by using barium-rich targets, post-annealing of the films, orchanging the stoichiometry of the targets, and it has been difficult toreproducibly fabricate high quality films.

[0005] There have been several attempts to overcome these difficultiesby using a buffer layer to produce RE123 films. For example, U.S. Pat.No. 5,232,900 describes the use of a non-superconducting buffer layer.U.S. Pat. No. 5,512,541 describes the deposition of superconductingfilms on a superconducting single crystal substrate. U.S. Pat. No.5,629,268 describes the deposition of multilayers on a bottomsuperconductor without damaging its superconducting properties. Kwon etal., Appl. Phys. Left., 62, 1289-1291 (1993), describe a buffer layer ofyttrium-doped Pr123 in order to improve the superconducting propertiesof ultra-thin Y123 films.

[0006] Despite these earlier efforts, the need to find additional buffermaterials suitable for reproducible deposition of various RE123 filmshas remained. Thus, an object of the present invention is buffermaterials suitable for reproducible deposition of various RE123 films.

[0007] Another object of the present invention is to simplify theprocess conditions needed to form various RE123 films.

SUMMARY OF THE INVENTION

[0008] To achieve the foregoing and other objects, and in accordancewith the purposes of the present invention, as embodied and broadlydescribed herein, the present invention provides a superconductivestructure including a dielectric oxide substrate, a thin buffer layer ofa superconducting material thereon; and, a layer of a rareearth-barium-copper oxide superconducting film thereon the thin layer ofyttrium-barium-copper oxide, the rare earth selected from the groupconsisting of samarium, gadolinium, ytterbium, erbium, neodymium,dysprosium, holmium, lutetium, a combination of more than one elementfrom the rare earth group and a combination of one or more elements fromthe rare earth group with yttrium, the buffer layer of superconductingmaterial characterized as having chemical and structural compatibilitywith the dielectric oxide substrate and the rare earth-barium-copperoxide superconducting film. Preferably, the superconducting materialcharacterized as having chemical and structural compatibility with thesuperconducting (rare-earth)-barium-copper oxide isyttrium-barium-copper oxide.

[0009] The present invention also provides a method of improving thesuperconducting properties of a superconducting(rare-earth)-barium-copper oxide structure by use of a thin layer of asuperconducting material characterized as having chemical and structuralcompatibility with the superconducting (rare-earth)-barium-copper oxideas a buffer layer situated directly between a dielectric oxide substrateand the layer of superconducting (rare-earth)-barium-copper oxide.Preferably, the superconducting material characterized as havingchemical and structural compatibility with the superconducting(rare-earth)-barium-copper oxide is yttrium-barium-copper oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 shows a schematic structure of the superconductingstructure of the present invention (layer thicknesses are not to scale).

[0011]FIG. 2 shows the superconducting transitions of superconductingsamarium-barium-copper oxide films, both with and without a buffer layerof superconducting yttrium-barium-copper oxide, as measured by ACsusceptibility.

[0012]FIG. 3 shows the current-voltage plots for superconductingsamarium-barium-copper oxide films, both with and without a buffer layerof superconducting yttrium-barium-copper oxide.

[0013]FIG. 4 shows a comparison of angle dependence (I_(c)/I_(co))between a superconducting yttrium-barium-copper oxide film and asuperconducting samarium-barium-copper oxide film including a bufferlayer of superconducting yttrium-barium-copper oxide in accordance withthe present invention.

DETAILED DESCRIPTION

[0014] The present invention is concerned with use of a buffer layer ofsuperconducting material on an underlying base substrate of a dielectricoxide substrate to improve the properties of a multilayer structureincluding the underlying base substrate, the layer of thesuperconducting material, and a subsequently deposited superconducting(rare-earth)-barium-copper oxide.

[0015] The base substrate in the present invention can be a dielectricoxide such as lanthanum aluminum oxide (LaAlO₃), strontium titanate(SrTiO₃), magnesium oxide (MgO), strontium aluminum tantalum oxide(Sr₂AlTaO6) or a solid solution of lanthanum aluminum oxide andstrontium aluminum tantalum oxide ((LaAlO₃)_(0.3)(Sr₂AlTaO₆)_(0.7) andneodymium gadolinate (NdGaO₃), or can be a composite material such ascerium oxide with a buffer layer of yttria-stabilized zirconia(CeO₂/YSZ), aluminum oxide with a buffer layer of cerium oxide(Al₂O₃/CeO₂) or silicon with a buffer layer. The base substrate may alsobe a composite including a flexible metallic substrate such as nickel,nickel-alloys and the like with a suitable buffer layer upon the metalsurface, such a buffer layer preferably being a material such as YSZ orMgO deposited by ion beam assisted deposition. Ion beam assisteddeposition is described in U.S. Pat. No. 5,650,378, U.S. Pat. No.5,432,151, and in allowed U.S. patent application Ser. No. 08/425,752,filed Apr. 19, 1995.

[0016] The buffer layer in the structure of the present invention is athin layer of a superconducting material that is chemically andstructurally compatible with a subsequently deposited superconductingfilm. Generally, the thin buffer layer is of a RE123 film where the rareearth element is selected from those rare earth elements that have arelatively smaller cation size such as for yttrium, erbium, ytterbiumand gadolinium. The RE123 film can be doped with a minor amount of,e.g., silver. Preferably, the thin buffer layer is of superconductingyttrium-barium-copper oxide, i.e., YBa₂Cu₃O_(7-x) or of superconductingsilver-doped yttrium-barium-copper oxide, i.e., Ag—YBa₂Cu₃O_(7-x). Thethin buffer layer is generally from about 5 nanometers (nm) to about 50nm in thickness, preferably from about 10 nm to about 20 nm.

[0017] By “chemical compatibility” is meant that the material does notundergo property degrading chemical interactions with the subsequentlydeposited superconducting (rare-earth)-barium-copper oxide. By“structural compatibility” is meant that the material has asubstantially similar lattice structure with the subsequently depositedsuperconducting (rare-earth)-barium-copper oxide.

[0018] The high temperature superconducting (HTS) material of thepresent invention can be any of the conventionally rare-earth-bariumcopper oxide (RE-Ba₂Cu₃O₇ or RE-BCO), where the rare earth metal is anindividual metal from the group of samarium, gadolinium, ytterbium,erbium, neodymium, dysprosium, holmium, lutetium, or is a combination ofrare earth metals such as, e.g., yttrium and samarium, yttrium andneodymium, yttrium and erbium, yttrium and holmium, yttrium andytterbium, yttrium and dysprosium, neodymium and samarium, neodymium,samarium and europium, neodymium, europium and gadolinium, neodymium,samarium, europium and gadolinium, and the like. The thickness of thesuperconducting top layer is generally from about 5 nanometers (nm) toabout 500 nm in thickness, preferably from about 10 nm to about 200 nm,although thicker films may also be employed.

[0019] The various material layers can be deposited by pulsed laserdeposition or by other well known methods such as evaporation,sputtering, or chemical vapor deposition. Pulsed laser deposition is thepreferred deposition method.

[0020] In pulsed laser deposition, powder of the desired material, e.g.,YBCO, can be initially pressed into a disk or pellet under highpressure, generally above about 500 pounds per square inch (PSI) and thepressed disk then sintered in an oxygen-containing atmosphere for atleast about one hour, preferably from about 12 to 24 hours. An apparatussuitable for the pulsed laser deposition is shown in Appl. Phys. Left.,56, 578(1990), “effects of beam parameters on excimer laser depositionof YBa₂Cu₃O_(7-x)”, such description hereby incorporated by reference.

[0021] Suitable conditions for pulsed laser deposition include, e.g.,the laser, such as a XeCl excimer laser (20 nanoseconds (ns), 308nanometers (nm)), targeted upon a rotating pellet of the desiredmaterial at an incident angle of about 45°. The target substrate can bemounted upon a heated holder rotated at about 0.5 revolutions per minute(rpm) to minimize thickness variations in the resultant film or layer.The substrate can be heated during the deposition at temperatures fromabout 600° C. to about 950° C., preferably from about 700° C. to about850° C., more preferably from about 700° C. to about 800° C. An oxygenatmosphere of from about 0.1 millitorr (mTorr) to about 10 Torr,preferably from about 100 mTorr to about 400 mTorr, can be maintainedwithin the deposition chamber during the deposition. Distance betweenthe substrate holder and the pellet can generally be from about 4centimeters (cm) to about 10 cm.

[0022] The rate of formation of the thin films or layers can be variedfrom about 0.1 Angstrom per second (Å/s) to about 200 Å/s by changingthe laser repetition rate from about 1 hertz (Hz) to about 200 Hz. Aslaser beam divergence is a function of the repetition rate, the beamprofile can be monitored after any change in repetition rate and thelens focal distance adjusted to maintain a constant laser energy densityupon the target pellet. Generally, the laser beam can have dimensions ofabout 3 millimeters (mm) by 4 mm with an average energy density of fromabout 1 to about 5 joules per square centimeter (J/cm²), preferably fromabout 1.5 to about 3 J/cm².

[0023] By use of the buffer layer of superconductingyttrium-barium-copper oxide, the superconducting properties of(rare-earth)-barium-copper oxides can be improved. For example, a 2000 Åthick samarium-barium-copper oxide film with a buffer layer ofsuperconducting yttrium-barium-copper oxide showed improvements intransition temperature as shown in FIG. 2 and in critical current asshown in FIG. 3 in comparison with a 2000 Å thick samarium-barium-copperoxide film without the buffer layer. FIG. 2 shows the comparison ofT_(c) as measured by AC susceptibility between samarium-barium-copperoxide films with and without the buffer layer upon a strontium titanatesubstrate. The samarium-barium-copper oxide film with the buffer layerhad a higher T_(c) and a sharper superconducting transition. As seen inthe data of FIG. 3, the samarium-barium-copper oxide film with thebuffer layer had a much higher critical current than without the bufferlayer.

[0024] The present invention is more particularly described in thefollowing examples, which are intended as illustrative only, sincenumerous modifications and variations will be apparent to those skilledin the art.

EXAMPLE 1

[0025] A series of structures with various substrates (LaAlO₃),(SrTiO₃), (NdGaO₃), and (CeO₂/YSZ) were formed both with and without abuffer layer of superconducting yttrium-barium-copper oxide and asuperconducting samarium-barium-copper oxide film. The thin films ofyttrium-barium-copper oxide and samarium-barium-copper oxide weredeposited onto the substrates by in situ pulsed laser deposition (PLD)using a 308 nm XeCl excimer laser and the same processing conditions. OnLaAlO₃ substrates, the substrate temperature range for deposition wasvaried between about 760° C. and about 815° C. with the highest T_(c)without the buffer layer obtained at 795° C. and at 10 MHz for adeposition rate of about 0.667 Å/s. Without the buffer layer, changingthe substrate temperature to 785° C. or 805° C., or changing the laserrepetition rate to 5 MHz dropped the T_(c) to from 89 K-91 K. With thebuffer layer, the superconducting samarium-barium-copper oxide film hada T_(c) in excess of 92 K within a lower deposition substratetemperature range of 765° C. to 785° C. and at laser deposition rates of5, 10 and 15 MHz. For the NdGaO₃ and CeO₂/YSZ substrates, the substratetemperature was 775° C. and laser repetition rate was 10 MHz for thesuperconducting samarium-barium-copper oxide film both with and withouta buffer layer of superconducting yttrium-barium-copper oxide.

[0026] The superconducting transition temperatures were measured for thevarious structures and the results are given in Table 1. TABLE 1Substrate T_(c) (no buffer layer) T_(c) (with buffer layer) LaAlO₃ 92.3K   93 K SrTiO₃   85 K 92.7 K NdGaO₃  <75 K 92.0 K CeO₂/YSZ 85.4 K 90.1K

[0027] In the case of LaAlO₃ substrate, high T_(c) was obtained withoutthe buffer layer in a narrow range of deposition temperatures anddeposition rates (frequency of laser pulse). By implementing the bufferlayer of the present invention, the optimal processing window where highquality samarium-barium-copper oxide films could be reproduciblyproduced was broadened significantly.

[0028] In the case of SrTiO₃ substrate, the highest T_(c) obtainedwithout the buffer layer in the range of deposition temperatures from765° C. to 790° C. and at deposition rates or frequency of laser pulseof 5 MHz or 10 MHz was 85 K. By implementing the buffer layer of thepresent invention, a higher T_(c) was obtained at a temperature of 775°C. and at 10 MHz. Again, the optimal processing window where highquality samarium-barium-copper oxide films could be reproduciblyproduced was broadened significantly.

[0029] While not wishing to be bound by the present explanation, theimprovement is believed due to the change of initial film growthchemistry for the samarium-barium-copper oxide film by theyttrium-barium-copper oxide buffer layer. Chemistry of other RE123materials containing large ionic size of RE³⁺ shows the possibility ofsubstitution of RE³⁺ for the barium ion (Ba²⁺) because of the similarityin size. The substitution disturbs the superconductivity and reducesT_(c). When samarium-barium-copper oxide is deposited on a dielectricoxide substrate, there is the possibility of the rare earth being in thebarium site and the amount of the substitution may vary depending uponthe deposition conditions and substrate (explaining the difference amongsubstrates in the values in Table 1). On the other hand,samarium-barium-copper oxide deposited on yttrium-barium-copper oxide,which has a small rare earth ion (Y³⁺) and does not have the problem ofyttrium substituting for barium, experiences different initial filmgrowth chemistry. The well-ordered yttrium-barium-copper oxide bufferlayer seems to prevent the substitution and to lock the position ofsamarium.

[0030] Further study of various buffer layers has been performed usingPr123 and homoepitaxially grown LaAlO₃. Pr123 has similar crystalstructure and lattice constants with a severe problem of thepraseodymium substituting for the barium. LaAlO₃ has shown passivationof a LaAlO₃ substrate for inborn defect structures. Neither of thesebuffer layers has been shown to improve either the T_(c) or the J_(c) ofa superconducting samarium-barium-copper oxide film.

[0031] Further study on properties of the structures of the presentinvention has compared the superconducting samarium-barium-copper oxidefilm with the yttrium-barium-copper oxide buffer layer on a substrateand a yttrium-barium-copper oxide layer on a similar substrate. Thesuperconducting samarium-barium-copper oxide film with theyttrium-barium-copper oxide buffer layer showed stronger pinning andimproved I_(c)(9 kOe)/I_(co), the normalized I_(c) in fields (H=9 kOe),as seen in FIG. 4.

[0032] Also, improved surface morphology has been found insuperconducting samarium-barium-copper oxide film with theyttrium-barium-copper oxide buffer layer in comparison to ayttrium-barium-copper oxide layer alone.

[0033] These results demonstrate that introduction of ayttrium-barium-copper oxide buffer layer between a rareearth-barium-copper oxide film and a dielectric oxide substrate canachieve improved T_(c) and J_(c) properties for a rareearth-barium-copper oxide film such as a samarium-barium-copper oxidefilm than in the absence of the buffer layer.

EXAMPLE 2

[0034] A structure with a substrate of LaAlO₃ was formed with a bufferlayer of superconducting silver-doped yttrium-barium-copper oxide and asuperconducting neodymium-barium-copper oxide film. The thin films ofsilver-doped yttrium-barium-copper oxide and neodymium-barium-copperoxide were deposited onto the substrates by in situ pulsed laserdeposition (PLD) at a temperature of 775° C. using a 308 nm XeCl excimerlaser. The thin film of silver-doped yttrium-barium-copper oxide wasdeposited from a target including 5 percent by weight silver. The oxygenpressure was 200 mTorr for the deposition of the silver-dopedyttrium-barium-copper oxide and 150 mTorr for the deposition of theneodymium-barium-copper oxide.

[0035] The superconducting transition temperature was measured as 89.7 Kfor this structure with a sharp transition width of only about 0.6 K.Another run under similar conditions had a superconducting transitiontemperature of 90.6 K with an equally sharp transition width.

[0036] These results demonstrate that introduction of ayttrium-barium-copper oxide buffer layer between a rareearth-barium-copper oxide film of neodymium-barium-copper oxide and adielectric oxide substrate can achieve high T_(c) properties for therare earth-barium-copper oxide film under broadened processingconditions. Previously, temperatures of at least about 850-900° C. hadbeen required to obtain superconducting transition temperatures greaterthan about 88 K for neodymium-barium-copper oxide.

[0037] Although the present invention has been described with referenceto specific details, it is not intended that such details should beregarded as limitations upon the scope of the invention, except as andto the extent that they are included in the accompanying claims.

What is claimed is:
 1. A superconducting structure comprising: adielectric oxide substrate; a thin buffer layer of a superconductingmaterial thereon; and, a layer of a rare earth-barium-copper oxidesuperconducting film thereon said thin layer of yttrium-barium-copperoxide, said rare earth selected from the group consisting of samarium,gadolinium, ytterbium, erbium, neodymium, dysprosium, holmium, lutetium,a combination of more than one element from said rare earth group and acombination of one or more elements from said rare earth group withyttrium, said buffer layer of superconducting material characterized ashaving chemical and structural compatibility with said dielectric oxidesubstrate and said rare earth-barium-copper oxide superconducting film.2. The structure of claim 1 wherein said buffer layer is ofyttrium-barium-copper oxide.
 3. The structure of claim 1 wherein saiddielectric oxide substrate is of a material selected from the groupconsisting lanthanum aluminum oxide, strontium titanium oxide, magnesiumoxide, neodymium gadolinium oxide, and a cerium oxide/yttria-stabilizedzirconia.
 4. The structure of claim 1 wherein said rareearth-barium-copper oxide is samarium-barium-copper oxide.
 5. Thestructure of claim 1 wherein said rare earth-barium-copper oxide isneodymium-barium-copper oxide.
 6. The structure of claim 3 wherein saidrare earth-barium-copper oxide is samarium-barium-copper oxide.
 7. Thestructure of claim 3 wherein said rare earth-barium-copper oxide isneodymium-barium-copper oxide.
 8. The structure of claim 1 wherein saiddielectric oxide substrate is of strontium titanium oxide.
 9. A methodof improving the superconducting properties of a superconducting(rare-earth)-barium-copper oxide structure by depositing a thin bufferlayer of a superconducting material characterized as having chemical andstructural compatibility with the superconducting(rare-earth)-barium-copper oxide directly onto a dielectric oxidesubstrate and depositing a layer of superconducting(rare-earth)-barium-copper oxide directly onto said thin buffer layer,whereby measured superconducting properties of critical current andtransition temperature for said superconducting(rare-earth)-barium-copper oxide structure are characterized as greaterin value for the structure including said buffer layer than for astructure of said superconducting (rare-earth)-barium-copper oxidewithout said buffer layer.
 10. The method of claim 9 wherein said bufferlayer is of yttrium-barium-copper oxide.
 11. The method of claim 9wherein said rare earth-barium-copper oxide is samarium-barium-copperoxide.
 12. The method of claim 9 wherein said rare earth-barium-copperoxide is neodymium-barium-copper oxide and said deposition is conductedat temperatures of from about 700° C. to about 800° C.
 13. The method ofclaim 1 wherein said dielectric oxide substrate is of strontium titaniumoxide.